Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of polymers, generally, they comprise (a) at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC) and/or an internal donor compound. ZN catalyst may also comprise (b) further catalyst component(s), such as a cocatalyst and/or an external donor.
Various methods for preparing ZN catalysts are known in the state of art. In one known method, a supported ZN catalyst system is prepared by impregnating the catalyst components on a particulate support material. In WO-A-01 55 230, the catalyst component(s) are supported on a porous, inorganic or organic particulate carrier material, such as silica.
In a further well known method the carrier material is based on one of the catalyst components, e.g. on a magnesium compound, such as MgCl2. This type of carrier material can also be formed in various ways. EP-A-713 886 of Japan Olefins describes the formation of MgCl2 adduct with an alcohol which is then emulsified and finally the resultant mixture is quenched to cause the solidification of the droplets. Alternatively, EP-A-856 013 of BP discloses the formation of a solid Mg-based carrier, wherein the Mg-component containing phase is dispersed to a continuous phase and the dispersed Mg-phase is solidified by adding the two-phase mixture to a liquid hydrocarbon. The formed solid carrier particles are normally treated with a transition metal compound and optionally with other compounds for forming the active catalyst.
Accordingly, in case of external carriers, some examples of which are disclosed above, the morphology of the carrier is one of the defining factors for the morphology of the final catalyst.
WO-A-00 08073 and WO-A-00 08074 describe further methods for producing a solid ZN-catalyst, wherein a solution of an Mg-based compound and one or more further catalyst compounds are formed and the reaction product thereof is precipitated out of the solution by heating the system. Furthermore, EP-A-926 165 discloses another precipitating method, wherein a mixture of MgCI2 and Mg-alkoxide is precipitated together with a Ti— compound to give a ZN catalyst.
EP-A-83 074 and EP-A-83 073 of Montedison disclose methods for producing a ZN catalyst or a precursor thereof, wherein an emulsion or dispersion of Mg and/or Ti compound is formed in an inert liquid medium or inert gas phase and said system is reacted with an Al-alkyl compound to precipitate a solid catalyst. According to examples said emulsion is then added to a larger volume of Al-compound in hexane and prepolymerised to cause the precipitation.
In polymerisation process this causes in turn undesired and harmful disturbances, like plugging, formation of polymer layer on the walls of the reactor and in lines and in further equipments, like extruders, as well decreased flowability of polymer powder and other polymer handling problems.
EP 1403292 A1, EP 0949280 A1, U.S. Pat. No. 4,294,948, U.S. Pat. No. 5,413,979 and U.S. Pat. No. 5,409,875 as well as EP 1273595 A1 describe processes for the preparation of olefin polymerisation catalyst components or olefin polymerisation catalysts as well as processes for preparing olefin polymers or copolymers.
Accordingly, although much development work has been done in the field of Ziegler-Natta catalysts, there remains a need for alternative or improved methods of producing ZN catalysts with desirable properties. One particular aspect in this connection is the desire to avoid as far as possible the use of substances which are considered as potential harmful compounds regarding health as well as environmental aspects. One class of substances which have been considered as potential harmful compounds is phthalates, which have been commonly used as internal electron donors in Ziegler-Natta type catalysts.
Although the amount of these phthalate compounds, used as internal donors in catalysts, in the final polymer is very small, it is still desirable to find out alternative compounds to replace phthalate compounds and still get catalysts having good activity, excellent morphology and other desired properties.
WO-1999024478-A1 discloses a polymerisation method to incorporate polymeric nucleating agents into the polymer to modify the mechanical properties.
This process is based on Ziegler-Natta catalysts as mentioned above, which comprise an internal donor based on phthalate-compositions.
However, some of such phthalate-compositions are under suspicion of generating negative HSE effects Furthermore, there is an increasing demand on the market for “phthalate-free polypropylene” suitable for various applications, e.g. in the field of packaging and medical applications as well as personal care, or personal hygiene.
WO2012007430 also incorporated herein by reference, is one example of a limited number of patent applications, describing phthalate free catalysts based on citraconate as internal donor.
However, up to now the mechanical properties of polypropylenes produced with catalysts having citraconate compositions as internal donors did not fulfill all the desired requirements, especially in view of stiffness/impact-balance. Up to now significantly higher amount of internal donor is to be used in the phthalate-free catalyst preparation to achieve the desired polymer properties
Although the inventors of WO2012007430 considered adding donor dissolved in organic solvent and TiCl4 to form washing solution, the reactivity of such catalyst is not satisfying in respect to activity (being low), or xylene solubles (being high).